The biblical laws of forbidden relationships transition here from direct blood relatives to a new category: women who become family through marriage. Once the boundaries of natural bloodlines are established, the focus shifts to protecting the sanctity of unions tied to close family members [רד צ הופמן].
A warning regarding an uncle is immediately clarified as a strict prohibition against his wife. Rather than presenting two separate offenses—such as a forbidden physical relationship with the uncle himself followed by an additional ban regarding his wife—the instruction is a single, continuous thought. The second part of the statement simply defines the first [רש״י, גור אריה, שפתי חכמים]. The absence of a connecting word between the phrases proves they form one unified idea [גור אריה]. In earlier laws regarding a father, other sources were relied upon to clarify that the restriction referred to the father's wife. Here, however, explicit detail was necessary to define the exact nature of the forbidden relationship [מזרחי, שפתי חכמים].
Most commentators emphasize that this restriction applies strictly to the wife of a father's brother. In contrast, the wives of a mother's brothers, or even a father's maternal half-brother, do not fall under this original biblical law, though they were later restricted by rabbinic decree [רלב״ג, ביאור יש״ר, רד צ הופמן, ברכת אשר]. This distinction stems directly from the biblical family structure. A father's brother belongs to the exact same family and tribe, making any relations with his wife deeply offensive. A mother's brother, however, belongs to a different family entirely, meaning his wife shares no essential, natural tie to the individual [שד״ל, רד צ הופמן]. While not every possible non-blood relative is listed, the text addresses the most common situations, relying on oral tradition to cover the rest [אבן עזרא].
The underlying reason for this law is that through marriage, the uncle's wife essentially becomes an aunt. Even without shared blood, her union elevates her to the exact same status as a biological sister of the father or mother [ביאור שטיינזלץ, רד צ הופמן, אבן עזרא]. This definition establishes two vital principles. First, the prohibition is permanent and remains in effect even if the uncle passes away [בכור שור, רד צ הופמן]. Second, it sets a broader legal standard that a wife is legally equated with her husband, a concept that influences other areas of law, such as the disqualification of relatives from testifying as witnesses [תורה תמימה].
Historically, it appears that restrictions based purely on a husband's family ties, rather than direct bloodlines, were entirely new concepts introduced at Mount Sinai. Before the Torah was given, such marriages were acceptable, as seen when Amram married his aunt, Jochebed [רד צ הופמן].